This Simple French Dish Is What Summer Tomatoes Are Made For

Name: This Simple French Dish Is What Summer Tomatoes Are Made For
Ingredients: 1 (14-ounce; 397 g) sheet puff pastry from 1 (17.3-ounce) package, such as Dufour, thawed if frozen; or 1 recipe Pâte Brisée; or 1/2 recipe Pie Dough; 1/4 cup (60 g) Dijon mustard or an equal-parts mixture of Dijon with crème fraîche (see note); 1/4 cup packed grated Gruyere cheese (1 1/2 ounces; 43 g); 1 medium tomato (9 ounces; 255 g), cored and sliced 1/4 inch thick; Kosher salt; Herbes de Provence, for sprinkling (optional); Extra-virgin olive oil, for drizzling